1. Field of the Invention
The present invention relates in general to methods for desegregating aggregate. In particular, the invention relates to methods for conveying and desegregating asphalt aggregate material from a delivery vehicle to a finishing machine.
2. Summary of the Prior Art
A. Prior Art Load Transfer Methods
The conventional method of laying an asphalt aggregate roadway mat includes the use of a finishing machine and a number of delivery trucks to transport the asphalt-aggregate material from the asphalt plant to the job site. Finishing machines of this type are called pavers and are well known to those skilled in the art. Such a finishing machine has a so-called floating screed at its rear end, usually provided with some form of vibratory means, to form the asphalt mat. The finishing machine also includes a hopper at its front end for receiving the asphalt aggregate material and suitable conveyor means, usually slat conveyors and screw augers, for delivering the material from the hopper to a position just in advance of the screed.
The technique for transferring the asphalt aggregate material from the dump truck to the hopper of the finishing machine normally requires the truck driver to position his vehicle immediately in front of the finishing machine, the latter being provided with rollers which engage the rear tires of the truck. During the time that the material is being transferred from the dump truck to the hopper of the finishing machine, the latter is advancing along the subgrade as it forms the asphalt aggregate mat and in doing so pushes the truck forwardly in tandem therewith.
A number of problems arise when operating the finishing machine and the delivery trucks in this manner. Quite frequently, the hopper of the finishing machine will be nearly empty but a loaded dump truck will not be available at the job site. When this occurs, the finishing machine must obviously stop and await the arrival of another delivery truck. When a finishing machine stops, even momentarily, the screed will tend to settle into the freshly laid mat. When the finishing machine then commences forward travel, the screed will tend to ride upwardly momentarily thus depositing an excessive amount of material. Consequently, such uneven surfaces are undesirable.
The failure of prior art methods to address this problem was addressed in U.S. Pat. No. 4,818,139 entitled "Methods and Apparatus for Making an Asphalt Aggregate Pavement." A self-propelled mobile apparatus is disclosed and claimed therein which has a large capacity hopper and a large capacity conveyor permitting a delivery dump truck to discharge its contents directly into the hopper in a rapid manner. The self-propelled vehicle may either shuttle between the finishing machine and a remote location of dump trucks, or travel in tandem with the finishing machine as the paving operation is performed. The self-propelled storage vehicle not only ensures that there will be a continuous supply of asphalt aggregate material to the screed of the finishing machine, but also remixes the asphalt aggregate in the large capacity hopper to desegregate it and thus improve pavement quality.
B. The Problem of Segregation
Segregation is a frequently recurring problem that has caused concern within the paving industry for decades and is receiving wide-spread attention by contractors, state highway departments, and equipment manufacturers. Segregation creates non-uniform mixes that do not conform to the original job mix formula. Segregation in a mixture results in a concentration of coarse materials in some areas of the paved mat, while other areas contain a concentration of finer materials. The resulting pavement exhibits poor structural and textural characteristics and has a shorter life expectancy. Elimination of segregation is essential to the production of high-quality paving mixtures. Segregation can be reduced by employing proper mix design. Mixes that are uniformly designed with no gap grading are generally very forgiving. However, gap graded mixtures are very unforgiving and in some cases simply cannot be produced without segregation occurring regardless of the techniques used.
Proper stock-piling techniques can also reduce segregation. Large stock piles are very sensitive to single aggregate blends. If the stock pile is formed using a conveying system, large particles roll to the outside of the pile causing segregated material to be fed to the plant. This problem can be reduced by making numerous piles of the aggregates rather than one large pile. But segregation can occur at numerous other points at a hot mix asphalt plant such as in cold-feed bins, hot bins on a batch plant, in the drum mixer, surge and storage bins, and during truck loading and unloading. Due to rapid truck loading underneath surge or storage bins at the plant, truck drivers often tend to pull the truck under the bin and not move it during loading. If the mix is sensitive to segregation, larger stones will roll to the front of the truck, to the rear, and to the sides as it would in a stock pile on the ground.
When unloading the truck into a paver hopper segregation again occurs, as coarse material accumulates at the outside portion of the paver wings. The use of a material transfer vehicle such as that disclosed in the above-referenced U.S. Pat. No. 4,818,139, overcomes the segregation problems that occur when material is unloaded from a dump truck into a paver, thus ensuring smooth pavement. Variable pitch augers in the bottom of the holding hopper remix and reblend the material as it discharges into the rear discharge conveyor. The reblended and remixed material is fed to the paver as the mat is being laid. Material transfer vehicles like this have one drawback: they are very large and more expensive. A compact vehicle which relatively inexpensively transfers material from a dump truck to the paver and at the same time eliminates segregation would be of great benefit.